1. Field of the Invention
The present invention relates to ink jet printer, and, more particularly, to printhead swath height measurement and compensation.
2. Description of the Related Art
An ink jet printer forms an image on a print media sheet by ejecting ink from an ink jet printhead. Typically, the ink jet printhead includes at least one columnar nozzle array made up of a plurality of individually selectable ink jetting nozzles that eject ink to form a pattern of ink dots on the print media sheet. Such an ink jet printer may include a reciprocating printhead carrier that transports one or more ink jet printheads across the print media sheet along a bi-directional scanning path defining a print zone of the printer. Typically, a mid-frame provides media support at or near the print zone. A sheet feeding mechanism is used to incrementally advance the print media sheet in a sheet feed direction, also commonly referred to as a sub-scan direction or vertical direction, through the print zone between scans in the main scan direction, or after all data intended to be printed with the print media sheet at a particular stationary position has been completed. Also, typically, the columnar arrays of nozzles of the ink jet printhead, when mounted to the printhead carrier, extend in a direction parallel to the sheet feed direction.
For a given stationary position of the print media sheet, printing may take place during one or more unidirectional scans of the printhead carrier. The term, unidirectional, often is used to refer to scanning in either, but only one, of the two bi-directional scanning directions. Thus, bi-directional scanning refers to two successive unidirectional scans in opposite directions.
The term, swath, refers to the area on the print medium traced by the printhead during a particular unidirectional scan of the printhead carrier where ink may be deposited. Thus, during the printing of a swath, individual printhead nozzles of the columnar nozzle array(s) trace along imaginary rasters spaced apart in the sheet feed direction and eject ink to form a printed pattern, such as for example printed lines, each line being formed by a plurality of ink dots. The swath height of a swath is determined, at least in part, by the extent of the columnar array of nozzles in the sheet feed direction, e.g., the distance between the top-most nozzle and the lower-most nozzle of the columnar nozzle array used in printing the swath.
Those working in the imaging arts continually strive to improve the print quality of imaging devices, such as ink jet printers. One such attempt is directed to reducing the occurrence of horizontal banding defects in printouts generated by an ink jet printer. Horizontal banding defects may be observed on print media, such as paper, as a horizontal white band. Such defects may be attributable to errors generated by the media sheet indexing mechanism that is used to advance a media sheet in a media feed direction through the printer during the printing of the text or image on the media sheet. Such errors can be caused, for example, by mechanical tolerances of the index roller and its associated drive train. It is known to mask such indexing errors by adopting an interlaced printing method, also referred to as shingling, wherein each scan of the printhead carrier (also sometimes referred to in the art as a printhead carriage) is made to vertically overlap a preceding scan. For a given swath, only a portion of the total print data for a given area on the print medium is printed. Thus, each scan of an actuated printhead produces a swath of printed output forming all or portions of multiple print lines, and multiple swaths may be required to complete the printing of any given print line.
Also, it has been recognized that banding effects may arise due to inaccuracies of the orientation or position of certain nozzles, particularly printheads manufactured using tape automated bonding (“TAB”) nozzle arrays. These effects may occur due to a concentration of aiming errors at the ends of the nozzle arrays, typically outboard-aimed nozzles as distinguished from the great majority of more centrally disposed nozzles. Some printers provide a built-in algorithmically operated automatic measurement of the effective increase of the pixel-swath dimension. This is followed by automatic adjustment of the printing-medium advance, typically extending the advance stroke by about half the extension of the swath dimension. However, such error is not always outboard and the swath-dimension change sometimes may be a contraction. Accordingly, the print-medium advance stroke may be shortened rather than lengthened.
One disadvantage of such a known in-printer approach is that the in-printer measuring system may lack sufficient accuracy and precision to properly compensate for swath height variations due to manufacturing variations in the nozzle spacings along the longitudinal extent of the columnar array of nozzles, from one printhead to another.
What is needed in the art is printhead swath height measurement and compensation for ink jet printing that addresses the shortcomings identified above.